Throughout her career, Kristen Stewart has had to hold her own against an array of costars: vampires; a Huntsman; Juliette Binoche. But in her new film, Personal Shopper, she does her best acting yet—and her scene partner is an iPhone. “That was Kristen’s joke,” says director Olivier Assayas. “She said, ‘You’re aware my costars on this film are my thumbs.’”

Stewart plays Maureen, a young woman convinced her recently deceased brother is trying to communicate with her. (She’s not just a personal shopper—she’s also a medium.) The first time she thinks she’s heard from him, it’s in the form of a text: “I know you,” the message from an unknown number says. “And you know me.” What transpires is a thread that propels the film’s Hitchcockian second half, during which Maureen goes from shake-sobbing with shock to flirting with the messages’ mysterious sender to seeing the texts as a sign of real danger.

The whole thing is surreal—not because it doesn’t seem possible, but because it does. More than any other movie before it, Personal Shopper shows the anxiety we all feel around texting, whether it’s with our significant other or a total stranger. When Maureen exhibits jittery dread over receiving a new message, or bites her nails as those little “typing” bubbles flash, she’s acting out something familiar to everyone who’s stared intently at their device.

If you combine the potency of social contact with the ambiguity that you get over text, it’s a really potent way to influence how somebody else is feeling, and what they’re thinking. psychologist Lisa Feldman Barrett

It’s also familiar to researchers who study the psychological effects of technology. It doesn’t really have a name, but texting has a very powerful effect on the brain. “When it’s uncertain whether someone likes you, dislikes you, or means you harm, your brain just can’t help but treat it like something important,” says Northeastern University psychologist Lisa Feldman Barrett, author of How Emotions Are Made. “If you combine the potency of social contact with the ambiguity that you get over text, it’s a really potent way to influence how somebody else is feeling, and what they’re thinking.” So when Stewart’s Maureen assumes an anonymous number is her deceased brother, it doesn’t feel ridiculous—it feels downright haunting.

That mental allure exists because the brain will always strive to fill in what it can’t know. It’s natural to think someone is conveying something implicit when they don’t respond to a text, whether that something is “I’m on my way to your house,” “I’m playing hard to get,” or even just “leave me alone.” Most of those imagined reponses have no basis in fact, but your brain creates them anyway, just to soothe your psyche until the truth emerges.

Sometimes that uncertainty leads to panic about why someone hasn’t texted back. Other times it leads to the presumption that a mysterious stranger is flirting. “When you don’t see the person you’re communicating with, it kind of fires up your imagination,” Assayas says. “It’s not that you’re connecting with an actual person, it’s more about projecting your fantasies onto another person who kind of agrees to be on the receiving end of them.”

Director Olivier Assayas and Kirsten Stewart on the set of Personal Shopper. IFC Films

Tech on Film

Not everyone agrees on whether Assayas was able to bring that entirely to screen—his movie got both cheers and jeers at Cannes. (Vanity Fair’s Richard Lawson did, however, write that “it’s one of the few times I’ve seen modern technology actually be scary instead of goofy in a movie.” He is correct.) But it does tap something far more real—psychologically and otherwise—than a lot of other movies that have tried to use technology to trigger our emotions. The “tongue phone” in Nightmare on Elm Street, the haunted VHS tape of The Ring, the call that came from inside the house in Scream: they were all scary, but none were as plausible as watching Stewart get a text that read “I have spares of your keys” from someone she couldn’t pick out of a lineup.

Personal Shopper, out today, also keeps it simple by focusing on just one side of the text exchange. Unlike shows like House of Cards or Pretty Little Liars that show texting just to depict dialogue, Assayas’ movie shows texting as it is: a semi-solo act that takes place over the course of days or weeks. IRL, someone texts, then goes about their day until they hear back, but in films and TV, texting is shown as something that takes place all in one scene. Assayas shows it how it really happens—interjected into daily life and influenced by events and emotions that come about while it’s happening.

Understanding the psychology of modern communication can lead to comedy, too. Larry Rosen, a psychologist at California State University, points to the Key & Peele sketch “Text Message Confusion,” which shows how our ability to misinterpret the ambiguity of text messages leads to one friend being ready to fight and another being ready to party. (Seriously, there are a lot of ways to read “let’s go.”) “It’s brilliant,” Rosen says. “It captured beautifully how we read between the lines when even the lines aren’t even clear.” (Jordan Peele channeled communication nervousness again in his directorial debut Get Out: Chris’ friend Rod interprets Chris’ lack of response as a sign that something is very wrong.)

The Text Stress

Ultimately, though, Personal Shopper—as with Get Out—shows the horror that texting can bring. That sounds frivolous, but it’s real. Electronic messages, especially ones from unknown and threatening sources, can have a truly damaging impact. “Can cyber-stalking or hacking or somebody tracking you down, can that be as nerve-wracking as someone following you or someone harassing you?” Barrett says. “The answer is: Yes, it can.” (For an example of this, check out the under-seen Ashley Benson vehicle Ratter.)

In a recent interview with V Magazine, Stewart said “when you speak to someone on the phone, that is a decipherable, understandable exchange. But with text and social media, it’s essentially a dialogue with yourself and your interpretation of a shadow.” Considering how the brain stresses out imagining things while waiting to receive the next text, she’s not wrong. And with each passing day, we add a new communication platform—Facebook Messenger, Tinder, Snapchat—to our lives and get a new shadow dance to interpret.

“Anything that connects you to people can also be used as a weapon against you.” Barrett says. “That’s the way things have always been, it’s just that now there are many more ways to be connected to people.”